390 



TUNICATA. 



These cells, which have been called by KOWALEVSKY inner foHicular 

 cells and by SALENSKY kalymmocytes, and as to the derivation of 

 which from the follicle-cells there can be no doubt, are homologous 

 with the test-cells of the Ascidians and the inner follicle-cells of the 

 Thaliacea (SALENSKY'S gonoblasts). According to SALENSKY, they 

 take a certain part in the formation of the embryo here as in the 

 Thaliacea. The statements on this subject, however, appear to us 

 somewhat inconclusive. 



An epithelial lamella further becomes separated from that part 

 of the inner surface of the follicle which lies next the oviduct (Fig. 

 184, ds) ; this covers the germ-disc like a cap and represents a 

 secondary germ-envelope that takes no further part in the develop- 

 ment of the embryo. This has been called by SALENSKY the cover- 

 ing layer. 



FIG. 183. A, lateral aspect of the egg of Pyrosoma, showing the first cleavage ; B t 

 the germ-disc of Pyrosoma at the six-celled stage, viewed from above (after KOWA- 

 LEVSKY). fz, inner follicle-cells. 



The cleavage of the egg of Pyrosoma, first made known through 

 the investigations of KOWALEVSKY, is discoidal and recalls that of 

 the Teleostei. The first stages seem to have a fairly regular course, 

 the germ becoming divided into two, four and eight blastomeres by 

 the successive appearance of meridional furrows (Fig. 36). The stage 

 of three blastomeres observed by SALENSKY and that of six found by 

 KOWALEVSKY (Fig. 183 B) must be regarded as accidental irregu- 

 larities. We have no further details as to the course of cleavage, 

 but its result is a so-called morula-stage (Fig. 1 84 B) in which the 

 germ-prominence is composed of blastomeres apparently irregularly 

 arranged and already forming several layers. 



The numerous inner follicle-cells (kalymmocytes) wander by means of 

 amoeboid movements into the spaces between the blastomeres (Fig. 184 A, fz) 

 and are even able to penetrate the cell-substance of the latter. This seems to 



